Upload & Sell: On
It's probably some sort of mechanical flutter -- either the shutter, or more likely the IS element on the stage of the shutter cycle. Notice the snow all along the bottom is blurry, as is the base of the largest tree on the far left. The next small tree to the right is also blurring, and moreso at its base. I'd like to see the entire image in extreme detail, piece by piece to note all affected areas.
Do you often shoot in near freezing conditions? Ever noticed the blur on other occasions? If it is a mechanical problem, I'm thinking there is a lubrication or hardening of some flexible piece causing this.
I've had IS blur affect photos, but that is always the entire image, not a section. So the shutter is involved somehow.
It may simply be the IS element started to move (vibrate) quickly at the beginning or end of the shutter cycle.
From experience (and I think some reading), 1/30 to about 1/100 is the "danger zone" for handheld IS shooting. The problem is you can be capable of holding the camera still enough that you are like a tripod -- fixed dampening of the vibration -- making the IS over-react and induce blur instead of stability.
That's my Photo CSI for the day! (show me blood splatters next!)